Search a 2D Matrix

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

• Integers in each row are sorted from left to right.
• The first integer of each row is greater than the last integer of the previous row.

For example,

Consider the following matrix:

[   [1,   3,  5,  7],   [10, 11, 16, 20],   [23, 30, 34, 50] ] 

Given target = 3, return true.

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        int xlen=matrix.size();
        if(xlen==0) return false;
        int ylen=matrix[0].size();
        if(ylen==0) return false;
        int index=xlen-1;
        while(index>=0 && matrix[index][0]>target)
            index--;
        if(index==-1) return false;
        vector<int>* v=&(matrix[index]);
        index=ylen-1;
        while(index>=0 && (*v)[index]!=target)
            index--;
        if(index==-1) return false;
        return true;
    }
};